A base station of a system which simultaneously supports a legacy system and an advanced system may operate in a mixed mode. Hereinafter, a mixed mode operation will be described on the assumption that a legacy system is an IEEE 802.16e system and an advanced system is an IEEE 802.16m system.
The mixed mode is divided into a WirelessMAN OFDMA zone (hereinafter, referred to as “L zone”: LZone) supporting a legacy (IEEE 802.16e) terminal and an advanced wireless interface zone (hereinafter, referred to as “M zone”: MZone) supporting an IEEE 802.16m terminal. In the mixed mode, an uplink (UL) zone may be divided into an L zone and an M zone using a time division multiplexing (TDM) or frequency division multiplexing (FDM) scheme. A ranging channel of an IEEE 802.16e system uses a diversity permutation type partial usage of subchannel (PUSC) structure and a ranging channel of an IEEE 802.16m system uses a subband contiguous resource unit (CRU) structure.
FIG. 1 shows an example of a TDD frame structure of a mixed-mode system applicable to an IEEE 802.16m system.
The frame structure shown in FIG. 1 is divided into a WirelessMAN OFDMA uplink/downlink zone and an advanced wireless interface uplink/downlink zone in downlink and uplink using a TDM scheme.
FIG. 2 is a diagram showing a TDD frame structure supporting a Wireless MAN-OFDMA uplink FDM operation.
The frame structure shown in FIG. 2 is divided into a WirelessMAN OFDMA downlink zone and an advanced interface downlink zone using a TDM scheme in downlink and is divided into an L zone and an M zone using an FDM scheme in uplink. That is, the WirelessMAN OFDMA downlink zone, the advanced wireless interface downlink zone and an uplink zone multiplexed using the FDM scheme correspond to a wirelessMAN frame having a length of 5 ms in a time domain. In case of the advanced wireless interface frame, the wirelessMAN OFDMA downlink zone may be regarded as a frame offset and a subsequent zone having a length of 5 ms may be regarded as a frame.
Unlike the system which operates using the TDM scheme as shown in FIG. 1, the system which operates using the FDM scheme as shown in FIG. 2 cannot use different permutation schemes. Thus, the IEEE 802.16m terminal must also use a ranging channel structure of the IEEE 802.16e system. That is, the system needs to be designed to allocate a ranging channel only to an L zone in consideration of overhead and to enable IEEE 802.16m terminals to share the ranging channel.